According to a prior art effort for elimination of phosphorus a part of the lake is cut off from the rest of the lake only leaving a small outlet. The idea is to achieve sedimentation of phosphorus containing algae in the smaller part of the lake only. However, it has turned out that the results are very moderate.
In many lakes which have received waste water there exists a sediment layer containing lots of phosphorus to a great extent in form of iron phosphate. The entry of waste water into the lake during its vegetation period results in growth of algae, particularly periphytic algae. These algae fall onto the bottom of the lake and provide energy for sulphate reduction through which hydrogen sulphide is formed. This hydrogen sulphide reacts with iron phosphate in the bottom sediment to form phosphorus acid and iron sulphide. If this is allowed to happen the lake will become entrophic or even hypertrophic. The release of phosphate is quite substantial. If 1 kg of algae reaches the bottom sediment about 100 kg of phosphate is released from the sediment.